docs: def set_completed(self, p_completion_date=date.today()):
          """
          Marks the todo as complete.
          Sets the completed flag and sets the completion date to today.
          """
          if not self.is_completed():
              self.set_priority(None)
  
              self.fields['completed'] = True
              self.fields['completionDate'] = p_completion_date
  
              self.src = re.sub(r'^(\([A-Z]\) )?',
                                'x ' + p_completion_date.isoformat() + ' ',
                                self.src)